1. Field of the Invention:
The present invention relates to improvements in a combustion chamber of a subchamber type internal combustion engine.
2. Description of the Prior Art:
At first, a known combustion chamber of a subchamber type of internal combustion engine in the prior art will be described with reference to FIG. 12. In this figure, a subchamber 2 is provided within a hollow cylinder head 4. The subchamber 2 is defined by an upper portion having a hemispherical shape and a lower portion having a frusto-conical shape or a cylindrical shape or the like. FIG. 12 shows the known subchamber defined by a lower portion having a frusto-conical shape. In the subchamber 2 are disposed a fuel injection valve 5 and, if necessary, a glow plug 6 for preheating the interior of the subchamber 2 upon the starting-up of the engine. The subchamber 2 communicates, via a subchamber injection port 3, with a main chamber 1 defined by the top surface of a piston 7, a cylinder 8 and the lower surface of the cylinder head 4.
Located along a cross-sectional plane of the subchamber injection port passageway passing through a subchamber center axis A--A and a cylinder center axis B--B, and extending within a subchamber mouthpiece 9 on the side of the subchamber injection port passageway wall located away from the cylinder center axis B--B, is defined a space 11 adjacent the subchamber injection port 3 and open to the subchamber injection port passageway. Within the space 11 are disposed a subchamber injection port control rod 14 and an expansible tube 12 connected with each other. Within the expansible tube 12 is sealingly enclosed a working substance (solid such as wax or the like, liquid or mixture of solid and liquid) that is subject to large amounts of expansion and contraction according to a temperature change thereof. A tip end portion 15 of the subchamber injection port control rod 14 lies nearly along the same plane as the subchamber injection port passageway wall surface or projects into the subchamber injection port 3. More particularly, the working substance such as wax or the like sealingly enclosed within the expansible tube 12 expands or contracts according to the temperature of the subchamber mouthpiece 9 and the temperature within the space 11. Accordingly, the length of the expansible tube 12 changes. Thus, the expansible tube 12 is a mechanism for controlling the cross-sectional area of the subchamber injection port passageway by actuating the subchamber injection port control rod 14 having the tip end portion 15. FIG. 12 shows the case where the control rod tip end portion 15 projects into the subchamber injection port 3 during a heavy loading operating state when the subchamber mouthpiece temperature and the temperature within the space 11 are high.
Next, the function of the above-described combustion chamber will be described.
Upon start-up or during a light loading operating state when the combustion chamber temperature is low and hence a subchamber mouthpiece temperature or the temperature within the above-mentioned space 11 is low, the tip end portion 15 of the subchamber injection port control rod 14 lies substantially along the same plane as the subchamber injection port passageway wall surface, and the cross-sectional area of the subchamber injection port passageway area is at a maximum. Accordingly, upon a compression stroke, the speed of the inflow of the gas within the main chamber 1 into the subchamber 2 created by compression of the gas within the main chamber 1 by means of the piston 7, and hence the speed of the swirl flow within the subchamber is small, and cooling of the fuel and thermal loss caused by the flow of gas is reduced. In addition, during a light loading operating state, since an injection amount of fuel is also small, the formation of a mixture of fuel and air within the subchamber as well as the combustion of the same can be insured. Also, upon an expansion stroke, the outflow of gas from the subchamber 2 into the main chamber 1 is facilitated since the cross-sectional area of the subchamber injection port passageway is large, and choking loss at the subchamber injection port is reduced. Furthermore, since the injection flow speed within the main chamber 1 is also small, thermal loss is also reduced.
As the load of the engine becomes high, the combustion chamber wall temperature, and hence the subchamber mouthpiece temperature and the temperature within the above-mentioned space, rises. And, since the working substance such as wax or the like within the expansible tube 12 expands, the length of the expansible tube 12 increases. Accordingly, the tip end portion 15 of the subchamber injection port control rod 14 connected to the expansible tube 12 projects into the subchamber injection port 3 and the cross-sectional area of the subchamber injection port passageway is reduced. In this case, as the load becomes higher, the amount that the tip end portion 15 projects into the subchamber injection port 3 becomes larger, and the cross-sectional area of the subchamber injection port passageway is further reduced. Consequently, as the load on the engine becomes greater, a jet flow speed from the main chamber 1 to the subchamber 2 during the compression stroke and hence a swirl flow speed within the subchamber 2 become high, resulting in the promotion of the mixing of fuel and air as well as the combustion of the mixture.
In addition, the jet flow speed to the main chamber 1 during the expansion stroke also becomes high. Hence, the mixing of the fuel and unburnt fuel within the main chamber 1 is promoted, and the combustion is improved.
However, in the above-described combustion chamber in the prior art in which a cross-sectional area of a subchamber injection port passageway is controlled by means of the expansible tube 12, the degree to which the stroke of the expansible tube is variable is small. If it is therefore contemplated to enlarge the degree to which the stroke is variable, the expansible tube 12 becomes very large and, due to the structure of the cylinder head, it becomes difficult to transfer to the expansible tube 12 a quantity of heat that is necessary to expand and contract wax as a result of the enlargement of the expansible tube 12.
In order to enlarge the stroke of the expansible tube 12, a number of turns of a screw-like part of the expansible portion (bellows) of the expansible tube 12 must be increased. Unless the diameter of the expansible portion is simultaneously enlarged, the value of the length of expansible portion/diameter of expansible portion becomes large and buckling would occur. Or else, if the degree to which the stroke of the expansible tube 12 is varied is increased by providing a relatively fewer number of turns of the screw-like portion and an increased pitch of the screw-like portion, then the expansible tube 12 must be large and fabricating the cylinder head becomes difficult.